This invention relates to a method for the direct metal plating of nonconductive substrates and, in particular, to the electrolytic metallization of through-holes in printed circuit boards without the need for prior electroless chemical) metal plating of the holes.
The metal plating of plastics and other nonconductive (dielectric) substrates is well-known in the art and the following description will be directed for convenience to plastics, especially epoxy, and the preparation of printed circuit boards.
The preparation of printed circuit boards requires the plating of conductive metal layers, usually copper, onto the plastic substrate of the board. These boards vary in design and may have a copper layer on each surface of the epoxy (two-sided boards) or they can be multi-layer boards which have a plurality of inter-leaved parallel planar copper and epoxy layers. In both types, through-holes are drilled in the board and metal plated to facilitate connection between the circuits on the copper layers.
In general and for purposes of illustration, the electroless metal plating method for the manufacture of through-hole printed circuit boards is a sequence of steps commencing with a commercially available copper clad laminate or multi-laminate made from, e.g., paper-epoxy or glass-epoxy material. A predesigned series of through-holes is formed in the board in any conventional manner, then preferably treated to clean and condition the plastic for adhesion and activation of the metal plating. In one process the board is first contacted with a solvent to swell the epoxy and enhance the effect of the subsequent oxidative, e.g., permanganate, etching step. After etching, the board is water rinsed and then neutralized with a reductant to solubilize manganese residues by lowering their oxidation state. The reductant preferably contains an acid fluoride to dissolve glass fibers in the epoxy. The board is now ready for electroless metal plating using conventional procedures.
A preferred step before plating, if not done previously, is to dissolve some of the glass fibers in the epoxy using, for example, acid fluorides, and then to pretreat the board by cleaning and conditioning with a composition such as ENPLATE.RTM. PC-475 to remove hydrocarbon soils and oxides and to enhance the surface for activation. Thereafter the board is immersed in a catalyst, such as a tin-palladium solution, which activates the surface of the epoxy for electroless copper plating. ENPLATE Activator 444 sold by Enthone-OMI, Inc., West Haven Conn., is exemplary of this type catalyst. Following a water rinse, the laminate is immersed in an accelerator such as ENPLATE PA-491 to activate the catalyst by freeing the metal palladium ions on the board. After a water rinse, the board is dried and is immersed in an electroless copper plating solution for a period of time sufficient to plate copper to the desired thickness on the board surfaces and the through-hole connections. ENPLATE CU-700 and other similar plating compositions may be employed. The boards may then be electroplated using conventional techniques to provide a thicker final coating.
The above procedure, however, is time consuming and costly and it is preferred to improve the process by eliminating the need for electroless copper plating of the board prior to electrolytic plating. This process is generally known as direct metallization and in a commercial direct metallization process, after the drilling and cleaning steps, the board is conditioned using an adhesion promoter such as an alkaline permanganate solution, followed by treating with a cleaner conditioner to remove oxides and oils. Activation is a two-step procedure consisting of a pre-dip to prevent contamination of the activator by drag-in followed by contacting the board with a proprietary noble metal containing catalyst which is adsorbed onto the board. The catalyzed board is now specially treated with a proprietary accelerator to leave a highly conductive and catalytic surface and a final acid wash stabilizes the conductive film. The board may now be directly metal plated by electrolytic plating. The board may also be electrolessly plated although for most applications this step is not performed and the board directly electrolytically plated.
Previous industry attempts to provide a direct metallization process include the use of an ultra-high levelling electrolytic copper bath, and substrate coatings such as a conductive polymer, a carbon rich material and copper sulfide.
It is an object of the invention to provide an improved direct metallization process for plating plastics.
It is a further object of the invention to provide a method for increasing the conductivity of a nonconductive substrate to enable the substrate to be either electrolessly or electrolytically plated.
A further object is to provide a conductive substrate article of manufacture which may be either electrolessly or electrolytically plated and the resultant plated product.
Other objects and advantages will be apparent for the following description.